Memristive And Negative Differential Resisitance Effects In ZnO/NSTO Heterojunction With Different Orientations

In recent years,with the rapid development of information technology,people have put forward higher requirements for information data storage,rapid classification and management.Memory is also constantly being updated,and most researchers expect to improve the performance and capacity of memory through new materials and new technologies.According to reports,researchers have found memristors can be applied in not only nonvolatile memory,but also brain-like neuromorphic computing and nonvolatile logical computing devices.Zinc oxide?ZnO?is one of the memristors compatible with conventional complementary metal oxide semiconductor technology,memristors based on ZnO also has the rectification characteristics and bipolar resistive switching effect which has been attributed to interface-type mechanism.Most memristors are conductive filaments,the formation/rupture process of conductive filaments has been observed through scanning transmission electron microscope,while the interface-type mechanism has the advantages of facilitating downsizing and improving integration.Furthermore,the memristive effect can be controlled by light,temperature,magnetic field and humidity,while the influence of crystal orientation has not been reported yet.In addition,there are few studies on the modulation of negative differential resistance effect?NDR?accompanying bipolar resistive switching,especially by negative pulse amplitude/width and compliance current.In this thesis,single crystal Nb:SrTiO₃?NSTO?substrates with a doping concentration of 0.7wt% were selected,and ZnO films were epitaxially grown by pulse laser deposition and magnetron sputtering.We found that different NSTO substrate orientations or pretreatment strongly affect the out-of-plane and in-plane orientation,morphology and electrical properties of ZnO thin films.Interestingly,negative differential resistance behavior was found to be strongly dependent on negative pulse width/amplitude,maximum positive?negative?dc voltage and compliance current.We found that there are great differences in the in-plane growth direction and electrical properties of the ZnO film grown on the?111?NSTO substrate with or without deionized water soaked.The out-of-plane orientation of ZnO films were both along the c-axis,while the in-plane orientation is strongly dependent on substrate pretreatment.Furthermore,a transition from rectification to bipolar resistance switching was found in ZnO films on unsoaked and soaked NSTO substrates,which is consistent with the increase of interface state density.The growth of ZnO thin films on?100?,?110?and?111?NSTO was also grown by magnetron sputtering technique,the differences in growth orientation and electrical properties of the device were also observed.ZnO thin films exhibit nonpolar?1120?orientation on?100?NSTO substrate with relatively high-interface state density,leading to the bipolar resistive switching and negative differential resistance.On the other hand,ZnO thin films exhibit polar?0002?orientation on both?110?and?111?NSTO substrates with relatively low interface state density.A typical rectification effect was found in both?0002?ZnO/?110?NSTO and?0002?ZnO/?111?NSTO heterojunctions.In addition,the relationship between the negative differential resistance effect of ZnO/?100?NSTO and the negative pulse width/amplitude,compliance current and maximum positive?negative?dc voltage is studied.The NDR reduces and shifts to a larger negative bias with increasing negative pulse amplitude/width,while it increases with no significant shift with increasing compliance current.The model of ionized oxygen vacancy and drift,combined with electron trapping/detrapping was adopted to understand the negative differential resistance behavior.